A well-known type of marine controllable pitch propeller comprises a pitch-adjusting mechanism in the propeller hub coupled to an inboard hydraulic cylinder by a force rod that extends through the propeller shaft. Usually, the control system for the propeller includes a feedback device that monitors the pitch of the propeller blades, and the inclusion of the feedback device accounts for the conventional use of the term "servo" to refer to the inboard hydraulic cylinder that actuates the pitch-controlling mechanism of the propeller. In a typical inboard servo installation, the servo is interposed in the propeller shaft aft of the gear box, although in at least one commercially available system the servo is built into the output gear of the gear box. In either case, the servo rotates with the shaft. To provide feedback to the control system, it is conventional to detect the longitudinal position of the force rod, which is indicative of the setting of the pitch-setting mechanism and, therefore, the pitch of the propeller blades. Because the propeller shaft and the force rod are rotating and the force rod moves axially within the rotating shaft, the feedback device commonly includes a coupling between the rotating force rod and a non-rotating feedback output element consisting of a special tubular coupling in the shaft having longitudinal slots of a length at least equal to the working stroke of the force rod, arms projecting from the force rod out through the slots, a rotating ring coupled to the arms and a follower riding in an external track on the ring.
U. S. Pat. No. 4,872,811 (Cavallaro et al., Oct. 10, 1989) describes and shows an inboard servo for a force rod-type marine controllable pitch propeller that incorporates a servo feedback arrangement and an emergency lock-up arrangement that work off a common feedback ring located aftwardly of the servo cylinder. Two connecting rods located generally symmetrically with respect to the axis of the force rod couple the feedback ring to the piston for conjoint movement therewith, the connecting rods passing through openings in an end wall of the cylinder in sealed relation. The emergency lock-up arrangement includes at least two threaded locking rods affixed to the cylinder and received freely through holes in the feedback ring in a generally symmetrical relationship with respect to the axis of the force rod and a locking nut received by each locking rod between the feedback ring and the cylinder and adapted to be threaded along the respective rod into engagement with the feedback ring. Upon such engagement, movement of the feedback ring is prevented, and consequently the piston cannot move in a direction away from the feedback ring because of the fixed connection between the feedback ring and the piston afforded by the connecting rods.
While the feedback and lock-up devices of the Cavallaro et al. patent are entirely satisfactory from a functional point of view and have several advantages over other designs, they are relatively expensive to manufacture, especially the feedback follower ring and its bearing and the rods associated with the feedback ring. Also, the rods that connect the feedback ring to the piston are exposed externally, which leaves open the possibility that they can be damaged; any "dings" in the portions that pass through the cylinder end wall are likely to cause leakage from the cylinder, which will make it necessary to replace the damaged rod, and the dings may also damage the seal, requiring its replacement as well.